It sounds like you are running out of memory. The problem may not be triggered by a specific app, but rather needing more memory or a combination of apps that consume a lot of memory.
You can determine what was going on at the time by taking a look at the audit logs. Turn on detailed audit logs in the QMC if not already on. Again, it may be that anything is the victim, not necessarily the cause when you hit the High WS level.
Here are some general things to check for tuning memory in the QMC Qlikview Server settings.
1. Make sure Document Timeout is set to a reasonable value like 180 minutes. The default of 480 minutes is too long for most cases.
2. Avoid using the preload option.
3, Make sure "Maximum Inactive Session Time" is set to a reasonable value.
4. "Possible Session Timeout:" should be less than "Maximum Inactive Session Time", generally half.
I'm working on some tools that profile server memory usage. If you contact me Contact - Rob Wunderlich Qlikview Consulting I may be willing to do some free diagnosis for you in exchange for you providing your log data for analysis.